1. Field of the Invention
This invention relates to a technique for generating a tomographic image of an imaging object using an optical coherence tomography technique.
2. Description of the Related Art
In technical fields of medicine and biochemistry, samples carried in an appropriate carrier such as cells and microorganisms cultured in a container are observed. Techniques for imaging cells and the like using a microscope or the like are proposed as methods for observation without affecting the cells and the like to be observed. One of such techniques utilizes an optical coherence tomography technique. In this technique, low-coherence light emitted from a light source is caused to be incident as illumination light on an imaging object and interference light of reflected light (signal light) from the imaging object and reference light having a known optical path length is detected, whereby an intensity distribution in a depth direction of the reflected light from the imaging object is obtained for tomographic imaging.
For a higher image resolution, it is thought to increase an NA (Numerical Aperture) of illumination light to be incident on an imaging object. By doing so, a resolution in a horizontal direction perpendicular to an incident direction of the illumination light can be improved. However, in an optical system having a large NA, a focusing range in a depth direction becomes narrower since a depth of focus (or depth of field) of the optical system becomes shallower. Thus, in the depth direction, a range capable of imaging with good image quality becomes narrow. In such a case, a tomographic image of the entire imaging object may be generated by splicing a plurality of partial images obtained at different focus positions in the depth direction.
For example, a technique for generating one tomographic image by splicing a plurality of focused images having a narrow observation range in a depth direction is described in the specification of International Publication No. 2012/128367. In this technique, to accurately align a plurality of focused images and synthesize an image, each focused image is arranged at a position having a highest correlation with an image in a wide range obtained using an optical system having a small NA.
In an optical tomographic imaging technique for performing imaging using coherence light as illumination light, random spot-like noise due to the interference of light reflected by fine unevenness of an imaging object is known to appear in an image. For example, if an imaging object is a cell or a cell cluster, the imaging object behaves as a semitransparent and irregular multi-layered scatterer. Thus, noise due to the interference of reflected light from each layer increases. Such a noise is called speckle noise.
Although the above conventional technique can obtain a tomographic image having a high resolution and a wide observation range, the influence of such speckle noise is not considered. Thus, there remains a room for improvement in terms of the quality of a tomographic image to be generated.